1. Field of the Invention
This invention relates to a horizontal raster width control apparatus which controls a horizontal raster width of a display apparatus equipped with a Cathode Ray Tube (CRT).
2. Description of the Related Art
FIG. 16 is a block diagram showing a configuration of a conventional horizontal raster width controlling apparatus. In the figure, reference numeral 1 denotes a horizontal output choke transformer, 2 denotes a horizontal output transistor, 3 denotes a dumper diode, 4 denotes a deflection yoke of a CRT, 5 denotes a capacitor which makes resonance with the deflection yoke 4, 6 denotes a capacitor for supplying supplemental energy for resonance, 7 denotes a diode rectifying an output voltage of a secondary winding of the horizontal output choke transformer 1, 8 denotes a capacitor for smoothing, 9 denotes a low pass filter removing ripples of the rectified and smoothed voltage with the diode 7 and the capacitor 8, 10 denotes a differential amplifier which outputs a signal indicating a difference between a horizontal raster width control signal from a D/A converter (which is not shown in the figure) and a raster width feedback signal outputted from the low pass filter 9, 11 denotes a horizontal deflection voltage control apparatus which controls an output voltage thereof according to an output signal from the differential amplifier 10.
Electric power for horizontal deflection is supplied to the deflection yoke 4 by the horizontal output transistor 2, dumper diode 3 and capacitors 5 and 6. The quantity of deflection current flowing through the deflection yoke 4 is controlled based on electric power supplied from the primary winding of the transformer 1. Namely, horizontal raster width is controlled according to a voltage outputted from the horizontal deflection voltage control device 11.
Further, the collector pulse voltage V.sub.CP applied to the collector of the transistor 2 increases in proportion to an increase in horizontal deflection current flowing through the deflection yoke 4. Therefore, the voltage rectified and smoothed by the diode 7 and capacitor 8 indicates a value of current flowing through the deflection yoke 4, i.e., horizontal deflection width. The voltage signal rectified and smoothed by the diode 7 and the capacitor 8 is inputted to the low pass filter 9 where frequency components higher than a predetermined frequency are attenuated. The voltage signal filtered by the low pass filter 9 is inputted to one of the input terminals of differential amplifier 10. A control destination value subject to correction of distortion of raster width in a vertical direction is inputted to the other input terminal of the differential amplifier 10 from a D/A converter (not shown). The difference signal between these input signals is produced by the differential amplifier 10 and feedback control of horizontal raster width is realized by varying the voltage +B outputted from the horizontal deflection voltage control device 11 to be applied to the horizontal output choke transformer 1 according to the difference signal.
The conventional horizontal raster width controlling apparatus is configured as explained above and it is very difficult to preset cutoff frequency of the low pass filter 9 because the parabolic modulation in a vertical synchronizing period for correcting the pincushion distortion is superimposed on the signal of the feedback system. Further, when a horizontal frequency is changed among a plurality of frequencies, or when system is in a transient state such as when the power of the apparatus is turned on or off, high voltage is applied to the horizontal output transistor 2 thereby the transistor has a lot of stress.